REPORT OF MR. ROBERT B. LINES.
483
Now, whenever the plunger T 2 is depressed, the sliding- weight on its rod presses on the tail of the index and prevents its descending, the index can only be brought down again and the semaphore released by a current from the station below when T 2 is up. This brings us to the third operation performed by the signal-man, which is to notify the station above, by unblocking his box-signal and releasing his semaphore, that the train has passed. The " up-line ” side being blocked on the diagram, the method of unblocking will appear if we trace the course of the circuit on that side from wire L 1 , P 1 , i 1 , L 1 , a , W 1 , through " down-bell” W 1 and the right coil of the electro-magnet E 1 to the ground. The alternate positive and negative currents move the escapement on the lower end of the armature, and the index, not being pressed by the sliding- weight, which would give it an upward bias, falls, by its own weight, tooth by tooth. White, or " line clear,” is shown behind the glass; the lever l is released by the half-turn of the axis, and this releases, as well, the spring rod G and pawl c, permitting the signal-man to turn his semaphore also to " line clear.”
From the above explanation it is evident that the signalman cannot advise the preceding station of the passage of a train until he has first blocked his own semaphore ; and that, having once blocked his semaphore, he cannot unblock it until he has received clear and unmistakable notice from the succeeding station that the train has passed there. Two motions—depression of the plunger and turning the handle of the inductor—are necessary for any signal, and, as a succession of alternately positive and negative currents is required to move the index, no signal can be delivered by lightning or other accident. For convenience, one coil (or rather one magnet; for the two coils are not connected as in ordinary electro-magnets) is used on each side for arriving, and the other for departing, signals.
To signal the forward movement of trains from one station
O
to the next, the upper plunger, say P 2 , is depressed and the handle of the magneto-inductor turned. The current then goes from the commutator marked T to P 1 , P 2 , and Z 2 , where it divides, a portion going out on the line, and the rest through L 2 , W 2 , up bell, and the left coil of E 2 to the ground.